1. Field of the Invention
The present invention relates to image processing method and apparatus for use in digital copying apparatuses, facsimile apparatuses, or the like which handle an image as an electrical signal.
2. Description of the Prior Art
There are what are called digital copying apparatuses in which, in general, an image is sampled by a CCD sensor or the like and the digitized data is outputted from a digital printer such as a laser beam printer or the like and thereby to reproduce an image. Such digital copying apparatuses are now widespread in place of conventional analog copying apparatuses owing to development of digital equipment. In such digital copying apparatuses, generally, gradation is reproduced by a dither method or a density pattern method to reproduce an image of a half tone. However, such methods have the following two large problems.
(1) In the case of an original of a mesh image, a periodic fringe pattern which does not exist on the original appears in the copied image.
(2) In the case of an original including line drawings, characters, or the like, the edge is cut at many positions due to the dither process, so that picture quality deteriorates.
The phenomenon of the above item (1) is called a moire phenomenon and it is considered that this phenomenon occurs due to the following causes (A), (B), and the like:
(A) Moire due to the mesh original and input sampling; and PA0 (B) Moire due to the mesh original and dither threshold matrix.
The moire phenomenon of type (A) is caused due to the occurrence of the beat frequency EQU .DELTA.f=.vertline.f-nf.sub.0 .vertline. [PEL/mm] (1)
This beat frequency .DELTA.f consists of the high frequency nf.sub.0 [PEL/mm] which is n times as large as the mesh frequency f.sub.0 (=1/P.sub.0) [PEL/mm] which is determined from the mesh pitch P.sub.0 [mm] of the mesh original and the input sampling frequency f.sub.s (=1/P.sub.s) [PEL/mm]]which is obtained from the input sensor pitch P.sub.s [mm].
On the other hand, when the threshold value of the dither is arranged as a dot concentration type such as a fatting type or the like, in general, the output image also has a pseudo-mesh structure, so that this causes beats between the input mesh original and the output image and the moire phenomenon of type (B) occurs. Assuming that the repetitive period pitch of the dither threshold value on the recording paper is P.sub.0 [mm], the frequency becomes F.sub.D =1/P.sub.D [PEL/mm] and the beat frequency becomes EQU .DELTA.f=.vertline.f.sub.0 -f.sub.D .vertline. [PEL/mm] (2)
Between the above-mentioned two moire phenomena of types (A) and (B), the moire phenomenon of type (B) most strongly occurs. This is because according to the phenomenon of type (A), in general, the value of n of the n-time high frequency of the mesh original is about 3 to 6 and the transfer function (MTF) or the like of the optical system or the like to lead the light to a sensor decreases considerably at that frequency, so that the contrast of the moire fringe is also low.
The moire phenomena which occur due to the foregoing causes make the quality of output image deteriorate remarkably. Therefore, various measures and investigations have been carried out hitherto. For example, according to a random dither method, the moire can be removed but a sand-like and grain-like image appears, so that the picture quality deteriorates. Paul G. Roetling has proposed ARIES in J. Opt. Soc. Am., Vol. 66, No. 10, Oct. 1976, page 985. According to this ARIES, the mean values of the densities before and after binarization are compared and the threshold value is controlled in a feedback manner so that they become equal. However, according to this method, the hardware is complicated and an effect sufficient to remove the moire is not obtained.
On the other hand, the remeshing method is described in Takashima, et al., "Meshing of Mixture Image of Characters and Photographs", Institute of Image Electronics, thesis 83-3, page 13. According to this remeshing method, the mesh image is remeshed by a dither pattern by means of a scumbling process (or averaging process with the peripheral pixels), so that the moire is removed and the grain-like noise is reduced.
However, the scumbling process (or averaging process with the peripheral pixels) results in a decrease in resolution. In other words, if one intends to remove the moire, the resolution decreases and if one intends to keep the resolution, the moire is not removed. Therefore, it is inevitable to preliminarily extract only the mesh image area and apply the foregoing method only to this area. For this purpose, what is called an image area separating technology is needed. According to this image area separating technology, the foregoing method cannot be easily realized since it is difficult to obtain the high-accurate and high-speed method, particularly, the method suitable to implement in hardware at the present technical level. Even if the image area separating technology is obtained, the high frequency component in the image is also averaged and smoothed by such a method, so that this method is still insufficient.
On the other hand, with respect to the problem (2), the characters and line drawings of the original are finely divided due to the dither process and, in particular, the edge portion is cut at many positions, so that the printing quality deteriorates. This phenomenon is particularly remarkable in the case where the dither pattern is the dot concentration type such as the fatting type or the like mentioned above.